The present invention relates to a hearing aid defined in the preamble of claim 1.
Hearing aids comprise an acoustic input aperture to receive ambient sounds and an acoustic output aperture to emit the ambient sounds that were processed in the hearing aid. It is of foremost importance that spurious acoustic signals—which subsequently would be processed as being ambient sounds—should not be superposed on these ambient sounds. Spurious acoustic signals may arise from airflows detaching off edges or in the vicinity of the hearing aid (detachment problems) or from airflows of different speeds and/or directions in the immediate of the microphone membrane which would cause this membrane to move (gradient problems). These two sources of spurious acoustic signals also may be encountered in combination.
The U.S. Pat. No. 4,073,366 describes a cover element which is bonded across the acoustic input aperture of a hearing aid. This known cover element of several layers bonded together at their edges consists of a porous material, its purpose being to preclude spurious acoustic signals generated by airflows detaching off the edges of the said input aperture.
It was found however that this known design only little affects the majority of the spurious acoustic signals: even though the cover element across the acoustic input aperture does reduce gradient-induced spurious acoustic signals, it fails to affect detachment noises. The reason is that the cover element mounted across the acoustic input aperture to reduce flow detachment by its own edges gives rise to likely new sources of spurious acoustic signals. In this design the source of interfering spurious acoustic signals only has been shifted.
Reference is made for the sake of completeness to the European patent document 0,310,866 which discloses covering the acoustic output aperture with a preferably microporous cover element to prevent ear wax from penetrating the hearing aid. Said document also discloses covering the acoustic input aperture with a cover element in case an in-ear hearing aid is involved because in such a case ear wax only might penetrate the hearing aid when latter is inside the ear. Steps minimizing noise interference cannot be inferred from this document which merely concerns the prevent of hearing-aid soiling.